Targeted Foliar Micronutrient Modulates Phenolic Accumulation in Olive Leaves

Olive (Olea europaea L.) leaf phenolics are increasingly valued across various industrial sectors. This study evaluated whether foliar application of copper (Cu), zinc (Zn), and manganese (Mn) can influence phenolic accumulation in olive leaves. One-year-old olive plantlets grown under greenhouse conditions were sprayed with these micronutrients. Samples of treated mature leaves were collected at 30, 60, and 90 days after treatment (DAT), whereas young leaves that developed after application were sampled at 90 DAT. Foliar fertilization significantly influenced the phenolic profile of olive leaves depending on the micronutrient applied, the sampling time, and the age of the leaves. The highest mean oleuropein concentration was recorded at 30 DAT (209.43 mg/100 g DW), driven mainly by the Cu × 30 combination (302.36 mg/100 g DW), whereas 90 DAT showed a significant decrease in most phenolic classes. In young leaves, oleuropein concentrations were an order of magnitude higher (2661–5105 mg/100 g DW) than those measured in mature leaves (14.5–302 mg/100 g DW). Furthermore, correlation analysis revealed nutrient-specific modulation of phenolic metabolism: Cu was positively associated with secoiridoids in mature leaves, Zn was positively associated with secoiridoids at 90 DAT, and Mn was negatively associated with flavonoids and with Cu and Zn at 90 DAT in young leaves. These findings suggest that foliar fertilization affects phenolic pathways differently depending on the applied micronutrient and the leaf developmental stage, providing new insights into the temporal regulation of olive leaf phenolic metabolism and identifying suitable harvest periods for the targeted phenolic compounds.